The prior art is particularly illustrated in United States patents U.S. Pat. No. 3,641,190, U.S. Pat. No. 1,470,359, and European patents EP-A-0 591 856, EP-A-0 143 486, EP-A-0 542 597 and EP-A-0 539 270.
In processes for the high temperature thermal transformation of hydrocarbons containing at least one carbon atom, for example pyrolysis between 900.degree. C. and 1500.degree. C. or steam cracking at about 850.degree. C. near the end of the heating zone, coke forms and deposits on the surface of the reactor walls. The reactor is then decoked, a process which is normally carried out in air at temperatures which are usually below 900.degree. C., attempting in the case of metal furnaces to avoid any overheating or hot spots which could damage the metal tubes of the furnace. Exothermic decoking thus requires the whole unit to be shut down and in particular, it requires the furnace to be disconnected from the downstream heat exchangers, reducing the total productivity of the unit. Further, safety regulations require the hydrocarbon introduction lines to be disconnected and replaced by air introduction lines, thus requiring a very long downtime for the unit.
The same disadvantages are there when rebuilding the unit for the pyrolysis phase, with the additional necessity of purging the reaction zone and lines with an inert gas.
Pyrolysis of hydrocarbons containing at least one carbon atom to produce olefinic or acetylenic hydrocarbons has been described, in particular in our patent applications FR-A-2 715 583 and FR-A-2 732 014, which are hereby incorporated by reference.
Pyrolysis reactors of ceramic material have been used in which non impermeable walls which are advantageously of ceramic material determine channels in which the feed and the reaction effluents circulate. These walls advantageously have a shape which is adapted to create turbulence and which, for example, comprise cells or cavities about the heating means. These latter are generally sleeves containing an electric heating element or a gas burner.